mm: memcontrol: remove unnecessary PCG_MEM memory charge flag

PCG_MEM is a remnant from an earlier version of 0a31bc97 ("mm:
memcontrol: rewrite uncharge API"), used to tell whether migration cleared
a charge while leaving pc->mem_cgroup valid and PCG_USED set.  But in the
final version, mem_cgroup_migrate() directly uncharges the source page,
rendering this distinction unnecessary.  Remove it.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>